All conventional coated abrasive backings, including those according to this invention, are normally converted to coated abrasive products by spreading a layer, substantially uniform in thickness, of fluid adhesive (called the "maker" or "make(r) coat" or "make (r) adhesive") on at least one major surface of the backing, dropping or electrostatically propelling graded abrasive grits into the adhesive layer, while it is still fluid, in such a way that the abrasive grits are substantially uniformly distributed over the adhesive coated surface of the backing, then solidifying the adhesive sufficiently to hold the abrasive grits in place. Usually another coat of fluid adhesive (called "size" "size(r) coat", or "size(r) adhesive") is spread over the layer of grits and solidified maker adhesive, and is then itself solidified, in order to improve the resistance of the abrasive grits toward becoming detached from the backing during use of the product.
Usually the process of solidifying either or both of the maker and size adhesives, or of some additional cloth finishing adhesive applied after the first one, involves heating. Heating can, and often does, cause backings to curl, because one major surface of the backing is lengthened or shortened to a greater degree than the other as a result of the heating. Sometimes, the curl disappears when the curled backing is cooled, and sometimes the curl remains, at least to some degree.
Curling of backings during processing, even if the curl disappears after cooling, is objectionable because it can make the product crack or stick to processing machinery, and can cause portions of a product web to adhere to other portions of the web. Such undesirable adhesions between different parts of the web are particularly likely to occur in the festoon ovens that are conventional in coated abrasive manufacturing, when a product curls severely, i.e., more than about 45 degrees, on its edges. (Throughout this application the extent of edge curl in a web is measured by the angle between an imaginary line tangent to the extreme edge of the curled web, in an imaginary plane perpendicular to the plane of the uncurled central part of the web, and a line, in the same imaginary plane, that forms the center line of the flat central portion of the web.)
Conventional cloth finishing adhesives are either resins that solidify by cross linking chemical reactions when heated, latexes that solidify by coalescence of small drops upon drying, or materials such as glue and starch that solidify because of gelation on drying. With these conventional adhesives, the long established practice of the art has achieved generally adequate control of edge curl, although it still causes occasional waste during manufacture of coated abrasive products.
More recently, cloth finishing adhesives that are solidified by chemical reactions induced by radiative energy input have been disclosed. For example, U.S. Pat. No. 4,474,585 of Oct. 2, 1984 to Gruber teaches in its Example 4 the use of a saturant or stabilizing adhesive that is susceptible to cure by UV light. U.S. Pat. No. 4,547,204 of Oct. 15, 1985 to Caul teaches the use of formulations suitable for curing by electron beam radiations.
Adhesives suitable for solidifying by exposure to radiation generally have unsaturated chemical bonds that can polymerize under the influence of radicals, cations, or anions formed by input of radiative energy. Acrylic acid and its derivatives are particularly suitable for radiation curing adhesives, and at least two types of these derivatives are in widespread use. "Acrylate(d) monomers" generally have a core of a di-, tri-, or higher poly-alcohol, usually of relatively low molecular weight, that has been esterified with acrylic acid or substituted acrylic acid to the maximum practicable extent. Typical commercial products in this class are trimethylol propane triacrylate (hereinafter TMPTA) and pentaerythritol triacrylate (hereinafter PETA).
"Acrylate(d) oligomers" are a more diffusely defined class. The cores of these products are generally oligomers of some relatively small molecule such as an isocyanate or epoxide. Depending on the terminal group of the oligomer, it may be converted to an acrylated oligomer by reaction with acrylic acid itself or with some derivative containing a suitable reactive group, such as hydroxy ethyl acrylate. The most common classes of acrylated oligomers are those made from epoxy resins of the bis-phenol A type, novolak phenolic resins, or ester linked urethanes. Suitable acrylated oligomers for coated abrasive finishing usually have average molecular weights per acrylate unit of 250-900. Acrylated oligomers are readily available commercially under such trade names as Novacure from Interez, Inc., Uvithane from Thiokol Corporation, Uvimer from Polychrome, Inc., and Purelast from Polymer Systems Corporation.
Adhesive formulations taught for coated abrasives, by all the published prior art known to applicants, often include trifunctional monomers such as TMPTA and sometimes include tetrafunctional monomers, but do not include monomers or oligomers of higher average functionality than four.
The present applicants have found that all the coated abrasive backings investigated by them with radiation curable cloth finishing adhesives as taught by the prior art are strongly susceptible to curl during subsequent processing, whether in further cloth finishing or in making and sizing, that includes cure of conventional resole phenol--formaldehyde resins with a molar ratio of formaldehyde to phenol of about 1.5. Overcoming curl under such conditions is a major object of this invention.